Another thought that I had while the thread was closed, is that eventually GOG.com will run into a wall licensing old PC games because they'll eventually run out of games that can be licensed without a huge amount of detective work... should GOG consider selling old console games? A lot of their DOS games use DOSBox already, so they are not averse to packaging emulators.
And NES games used to be licensed to PC when GameTap was an extent service. So it's not a matter of Nintendo saying NES/SNES games can't be sold on PC at all -- there are already SNES games on Steam (Legend & Jim Power being two of them).
I think GOG definitely should consider at the very least tapping into the back catalogues of older computers like the Amiga, Commodore 64, Spectrum, etc. But also consider the NES, SNES, SMS, etc. Get in touch with third party companies, as well as Hamster who licenses a lot of old games.